1. Field of the Invention
The present invention relates to an improved nonaqueous electrolyte secondary cell using a negative electrode active material such as lithium, lithium alloys, or a carbon material intercalatable with lithium; an electrolyte such as a solution with a nonaqueous solvent; and a positive electrode active material such as a compound oxide containing lithium. More particularly, it relates to a process for producing a positive electrode active material having a hexagonal crystalline structure represented by the general formula, Li.sub.y Ni.sub.1-x Me.sub.x O.sub.2 where Me is any one of Ti, V, Mn and Fe) with lattice constants, a.sub.o =2.83 to 2.89, c.sub.o =14.15 to 14.31 .ANG. as identified from X-ray diffraction pattern, and an nonaqueous electrolyte secondary cell using the same.
There has been a vigorous demand of miniaturized secondary cells having a light weight and a high energy density to be used as an electric supply for driving electronic apparatuses such as audio and video appliances and personal computers which have been rapidly rendered portable or cordless in recent years.
For this reason, nonaqueous secondary cells, particularly, those using lithium as active material have been strongly expected to be secondary cells having specifically high voltage and high energy density.
2. Description of the Related Art
As positive electrode active materials to meet the abovementioned demand, there have been proposed layered compounds capable of being intercalated and deintercalated with lithium such as compound oxides containing main components of lithium and transition metals (referred to as lithium compound oxides hereinunder), for example, Li.sub.1-x NiO.sub.2 with 0.ltoreq.x&lt;1, (U.S. Pat. No. 4,302,518); Li.sub.y Ni.sub.2-y O.sub.2 (Japanese Patent KOKAI (Laid-open) No. 2-4302518); Li.sub.y Ni.sub.x Co.sub.1-x O.sub.2 with 0&lt;x.ltoreq.0.75, y.ltoreq.1 (Japanese Patent KOKAI (Laid-open) No. 63-299056). In other materials which have been proposed heretofore there are a compound oxide, A.sub.x M.sub.y N.sub.z O.sub.2 where A is an alkali matal, M is a transition metal, and N is at least one of Al, In and Sn with 0.05.ltoreq.x.ltoreq.1.10, 0.85.ltoreq.y.ltoreq.1.00, 0.001.ltoreq.z.ltoreq.0.10 (Japanese Patent KOKAI (Laid-open) No. 62-90863); and a combination of a major active material, Li.sub.x M.sub.y N.sub.z O.sub.2 where M is at least one of transition metals and N is at least one of non-transition metals with 0.05.ltoreq.x.ltoreq.1.10, 0.85.ltoreq.y.ltoreq.1.00, and 0.ltoreq.z.ltoreq.0.10 and a sub-active material, i.e., a lithium-copper compound oxide (Japanese Patent KOKAI (Laid-open) No. 4-2066).
Synthesis of the positive electrode active materials have been achieved, for example, by heating at temperatures in the range of 600.degree. to 800.degree. C. in air for Li.sub.y Ni.sub.2-y O.sub.2 as disclosed in Japanese Patent KOKAI (Laid-open) No. 2-40861, or by heating at 900.degree. C. for 5 hours in air for Li.sub.y NiCo.sub.1-y O.sub.2 as disclosed in Japanese Patent KOKAI (Laid-open) No. 3-49155.
Japanese Patent KOKAI (Laid-open) No. 4-181660 proposed that the synthesis of LiMO.sub.2 where M is one or more selected from Co, Ni, Fe and Mn should be achieved by heating at temperatures in the range of 600.degree. to 800.degree. C., preferably by effecting twice the treatment at 800.degree. C. for 6 hours. Alternatively, Japanese Patent KOKAI (Laid-open) No. 4-24831 proposed that the synthesis of A.sub.x M.sub.y N.sub.z O.sub.2 where A is an alkali metal, M is a transition metal and N is at least one of Al, In and Sn with 0.05.ltoreq.x.ltoreq.1.10, 0.85.ltoreq.y.ltoreq..ltoreq.1.00, and 0.001.ltoreq.z.ltoreq.0.10 should be achieved, for example, by heat-treating at 650.degree. C. for 5 hours, and then heat-treating at 850.degree. C. for 12 hours, both in air.
EP 468 942 (A2) proposed that the synthesis of Li.sub.x Ni.sub.2-(x+y) M.sub.y O.sub.2 where 0.8.ltoreq.x.ltoreq.1.0, and M is any one of Co, Fe, Cr, Ti, Mn, and V, should be accomplished by dispersing nickel hydroxide in a stoichiometric excess of a solution of lithium hydroxide into a slurry, drying the slurry by spray drying, and then heat-treating at a temperature of 600.degree. C.
With these active materials, development of practical high energy density secondary cells having a discharge voltage on the order of 4 V is being proceeded.
Among these positive electrode active materials, for example, Li.sub.1-x NiO.sub.2 where 0.ltoreq.x&lt;1, (referred to as LiNiO.sub.2 hereunder) exhibits a potential of 4 or more relative to lithium, so that its use as a positive electrode active material allows a secondary cell having a high energy density to be realized. The charge and discharge characteristics of the cell, however, are deteriorated with increasing the number of cycles though a discharge capacity of not less than 100 mAh/g can be obtained at initial cycling stage, reaching 65% of the initial capacity after 50 cycles. Thus, there is a problem of impossibility of achieving a good discharge and discharge cycle property. There is another propose to produce a cell having an excellent cycle property by synthesizing a compound oxide represented by the aforementioned general formula, but modified by using nickel as transition metal, a part of which is replace by non-transition metal of indium, aluminum or tin, thereby obtaining an improved positive electrode active material. However, such a lithium compound oxide as containing an amount of nickel partially replaced by the aforementioned elements tends to reduce the discharge voltage, which adversely affects the characteristics of high voltage and high energy required essentially for the cells.
The charge and discharge capacity of the active materials having this type of layered structure is attributed to an great extent to the crystalline structure of the synthesized active materials. That is, if a desired crystalline active material comprises entirely such a layered crystalline structure as belonging to the space group of Rm, the maximum capacity in charging and discharging can be achieved. In most cases, however, crystalline domains having a rocksalt structure belonging to the space group of Fm3m are formed in the course of the synthesis. Thus, the domains of the rocksalt structure are produced when an insufficient amount of oxygen is provided in the thermal diffusion of the lithium during the synthesis, or when insufficient thermal vibration or insufficient period of time for the reaction to cause sufficient diffusion of the lithium into the crystalline matrix is bestowed on the lithium.
The presence of such domains dimimishes extremely transfer and diffusion of the lithium ions and the number of receptive sites therefor, resulting in producing a reduction in the capacity in charging and discharging. For the reasons as described above, it is difficult to produce the active materials having the structure of the space group of Rm which allows for a higher capacity.
For example, even if the LiNiO.sub.2 is subjected once or twice to heat-treating at a temperature of 600.degree. to 800.degree. C. for 10 hours in an atmosphere of air, as has been proposed heretofore, one can not obtain a crystalline structure comprising the perfect space group Rm as identified by the X-ray diffraction pattern shown in FIG. 1. Thus, the ratio in the peak intensity of the face (104) to the face (003) of Miller indices is higher than 1 and similarly the ratio in the peak intensity of the face (102) or (006) to the face (101) as shown in FIG. 1 is higher than 1, which are greately different from those of the structure consisting predominantly of the space group Rm as shown in FIG. 2. In addition, with respect to the lattice constants, crystal lattice parameters, a.sub.0 is 2.885 .ANG. and c.sub.0 is 14.192 .ANG. in FIG. 2 while a.sub.0 is 2.905 .ANG. and c.sub.0 is 14.235 .ANG. in FIG. 1 indicating an expansion behavior of the lattices.
From the expansion of the lattices and the difference in the peak intensity ratio as described above, it may be considered that the crystalline structure is distorted in the presence of the mixed domains of both the space groups, i.e., the Rm and the Fm3m resembling to the former in crystal parameters. Therefore, there is given a problem that the aforementioned synthetic processes can not achieve such active materials capable of providing sufficient capacity in charging and discharging.